The present invention relates generally to draft control systems for tractor or vehicle drawn implements and more particularly to an improved electronic control system for automatic position control and/or draft control of the implement with respect to the vehicle to which it is attached.
One of the most common present day systems for connecting an implement to a tractor is known as a three-point hitch. The three-point hitch consists of a pair of draft arms that provide the connection between the implement and the tractor and a rock arm connected to a rock shaft and also to the draft arms through links. The rock shaft is positioned by a hydraulic motor. Fluid is selectively admitted to the hydraulic motor through a valve which is actuated in response to changes in the draft forces on the draft arms, the manual positional setting of the system and the position of the rock shaft. Heretofore, most of the systems that are commercially available consist of a strictly mechanical sensing system for sensing changes in draft load, the position of the rock shaft and the position of a manual control lever. These mechanical systems become extremely complicated and require a substantial amount of maintenance.
As such, many recent efforts have been directed toward development of an electronic system for controlling the hydraulic valve which supplies fluid to the hydraulic motor. Examples of such devices are shown in U.S. Pat. Nos. 3,516,498 and 3,814,188.
So far as presently known, most systems that have been proposed rely primarily upon signal amplification for measuring changes in the various parameters of the system. In other words, a signal is developed from the various sensing devices, and the signals are combined and amplified to produce an output signal. When the combined output signal exceeds a certain level, a control valve is actuated to make a correction. However, these systems have lacked reliability and the requisite sensitivity to measure small changes in the respective parameters.